Telephone transmission



June 16, 1931. 0. E. BUCKLE Y ET AL 1,809,823

v TELEPHONE TRANSMISSION Filed Feb. 21, 1929 2 Sheets-Sheet 2 FIG. 2

I I I l 0 5oomoo I500 2000 2500 A TTORNEV Patented June 16.1931

UNITED STATES. P T NT orrics OLIVER E. BUGKLEY, OF MAPLEWOOLD, AND'RALPI-I V. IQHAR'ILEY, OF SOUTHORANGE, NEW JERSEY, ASSIGNOBS TO BELLTELEPHONE LABOBA'IORIEiS, INCORPORATED, OF

NEW YORK, 1v. Y., e oonronsrion on NEW YORK TELEPHONE TRANS/MISSION oApplication fil d February, 21 1929. 7 Serial No. 341,623.

The present invention relates to signaling systems and particularly tothe transmission of speech currents overv long submarine cables.

An object of the invention is to providefor the efficient, transmissionand intelligible receptionof speechover a submarine signaling,

cable Where the sending current is limited by the physicalcharacteristics of the cable, and

the minimum practicable value of received current in any'frequencyregion is established bythe level of extraneous interference in thatfrequency region.

Another object of the invention is to utilize most efficiently a loadedsubmarine signaling cable of great attenuation having a variableattenuation through a wide range of the frequencies of thespeechspectrum for the transmission of speech to great distances. y

A further objectof, the invention is toprovide for transmission oftelephone currents over a loaded signaling conductor. of great lengthdefinite diameter, and definitephysical characteristics, the length andconstants of the cable being such that the transmission of commercialspeech overit necessitates the application of more advanced techniquethan heretofore employed for telephone transmis SlOIl. w

In accordance with one feature of the present invention the effect ofmodulation on the loadedca'b-le because of the non-linear currentinductance curve ofthe'cable makesit desirable to eliminate a wide rangeof the lower cies up to I00 cycles, moreorless, may be practically orwholly suppressed with no' decrease or perhaps an actual gain in-theintelli .gibility of the speech transmitted over the cable. The resultsof this are'several fold in that the cable is not burdened with,powero'f the lower frequencies, whichmakes it possible to impress uponthe cable more energy of higher frequencies which are more usefuli forintelligibility; in that the'production of har- 'monics of the lowerfrequencies is prevented,

thereby preventing the partial masldng of the higher frequencies by theharmonics; and in that the production of combination tones of thefrequencies below 400 cycles with those above 400 cycles is prevented.In cases .where thetransmission of I frequencies below 400' 0 cyclesispermissible, the band of frequencies from zero to 400 cycles may beused for simultaneous telegraphy. p r

In accordance with another feature of the present invention, the speechwaves are predistorted before impression upon the cable in such a mannerthat the average energy of any frequency impressed upon the cable,

.when attenuated by the cable an amount corresponding to the cableattenuation at that tortingthespeech waves to be impressed upon thecable at the transmitting end in sucha fashion .thatthe higherfrequencies in the speech band are transmitted with a greatly increasedamplitude with respect to the lower frequencies; Thismay be accomplishedby attenuating the lowerfrequencies to thedesiredlevel as compared withthe higher frequenciesand by entirely suppressing the speech frequenciesbelow about 400 cycles and those above about 2500 cycles, because at thereceiving end the amplitude of the frequencies above about 2500 cycleswill be smaller (or so little greater) than the amplitude of the noisecurrents that they consequently can'- not ibe' efficiently received. Theremaining waves are'impressed upon the cableat the neticproperties'ofthe loading'material or speech frequency spectrum is then amplified tothe desired sending level andrthe signaling dielectricpropertiest of thecable insulation.

g-The waves transmitted in this manner when received at the distant endand uniformly amplified will not be of sufficient intelligibility, orifintelligible, the speech will not be natural, Hence in addition tosupplying a verygreat amplification at the receiving end,

it 1s necessary'to apply a correcting distortion or attenuationequalization, whereby the relative amplitudes of the various frequencycompone-nts between about 400 cycles and 2500 cycles are made toapproximatemore closely those of normal speech.

Consider a cable about 2000 miles long continuously or otherwise loadedwith mag netic material and having an attenuation of 80 decibels (hereinabbreviated db.) at 400 cycles and about 120 db. at 2000 cycles andhaving at the receiving end a normal volume and a normal distribution ofnoise. The problem dealt with in the present specification relates tothe transmission from one end to the other and reception of speech oversuch a cable.

The explanation of how commercial speech may be transmitted and receivedover such a cable in one particular instance will be given in connectionwith Figs. 1 to 3, Fig. 1 being a diagrammatic representation of afour-wire circuit for connecting a land line to a cable;

Fig. 2 being graphs on a logarithmic scale of the power-frequencydistribution of the signal waves at various points along the systemprior to impression upon the cable and Fig. 3 being corresponding graphsat various points along the system at the receiving end of the cable.

' In Fig. 1, S indicates a substation including a microphone transmitter9 transmitting speech power through transformer 10 to land line 11 andits balancing network BN over hybrid coil arrangement 12 to band passfilter 13. The four-wire circuit with hybrid coil and network balancingthe cable is a typical representation of apparatus duplicated at thedistant end of the cable. For the purpose 0 this description we assumethat the speech power impressed upon filter 13 is of practicallyconstant volume. The following discussion will be based upon the averagepower distribution of various voices when the current of each voice isreduced to an equal volume. Filter 13 is adapted to suppresssubstantially speechwaves having a frequency below about 400 and thoseabove about 2500 cycles per second. The curve a of Fig. 2 shows on alogarithmic scale the frequency-power distribution in the air of thesound waves impressed upon the microphone 9; Curve 7) shows thefrequency-power distribution of the speech currents after transmissionthrough the land line. Curve 1) depicts the resultant after passing thewaves through filter 13. In order to restrict the variations of thespeech currents and to increase the amplitude thereof they are passedthrough two attenuation-equalizer-amplifiers 14 and 15 in closesuccession. The device 14 substantially flattens out the frequency-powerdistribution curve and amplifies the speech currents as shownin curve 0.The device 15 predistorts the frequency spectrum and gives itsubstantially the shape shown by curve a. Amplifier 16 uniformlyamplifies the speech current to a level such as represented by curve d.Devices 14, 15 and 16 are preferably chosen so as to insert an over allgain of about 80 db. The amplified currents are then impressed uponhybrid coil 17 and from thence upon the cable 18 at the highest levelpermitted by the magnetic properties of the loading material and thedielectric properties of the cable insulation.

A cable of a length of 1000-2000 nautical miles introduces a greatattenuation greatly varying with frequency and distorts the transmittedspeech waves in a corresponding manner. Curve 6 of Fig. 3 depicts thefrequency-power distribution of the waves received at the receiving endand curve f illustrates the noise level over the range 400 to 2500cycles of a typical cable under average conditions. The distance betweencurves e and f for any particular frequency range may be varied asconditions will permit or as intelligibility considerations mayindicate. It is known that certain ranges of frequencies contribute morethan others to speech intelligibility and these ranges may be favored.In order to render the received speech intelligible'it is first passedthrough amplifier 19 which inserts a gain of about 50 db. and therebyraises its level from that of curve e to that of curve g and the noiselevel from curve f to curve h, Fig. 3. Band pass filter 20 suppressesall noise currents having frequencies below about 400 and above about2500 cycles per second. Attenuation-equalizer-amplifier 21 shapes thecurve of the received speech waves to substantially approximate f thecurve z, the curve is representing the correspondingly amplified andshaped noise level. The curve m represents the curve of the acousticpower which will finallybe delivered by the receiver at the distant endand n the curve of the corresponding noise level.

The equalizing amplifiers 14:, 15 and '21 may consist of severalamplifying stages and one or several equalizing networks as may bedesired. Likewise amplifiers 16 and 19 may consist of any desired numberof thermionic tubes with appropriate input and output impedances andcurrent supply sources. The equalizing networks may be designed inaccordance with known principles such as set forth in U. S. patent toZobel 1,603,305, October 19, 1926; Blackwell U. S. Patent 1,454,011, May1, 1923; Johnsons Trans mission Circuits for Telephonic Communication(Van Nostrand N. Y.1925) especially Chapter XVIII; and other patents andpublications.

The relative and absolute magnitudes of amplification, attenuation, andenergy distribution given herein are those which may be encountered in atypical case and each case is governed by a particular set of conditionsfor which due allowance should be made.

What is claimed is:

1. A method of telephone transmission over a system comprising a landline and a submarine cable characterized in this that the lowerfrequencies of the speech currents ing a combined transmissioncharacteristicwhereby the high frequencies are amplified more than thelower frequencies, and whereby after. attenuation by the cable theaverage amplitude of the received waves isabove the average receivednoise level at that frequency.

3. A system in accordance with claim 2 characterized by the provision ofmeans at the receiving end restoring to a considerable" extent, atleast, the normal frequency-amplitude relation of the voice waves beforeapplying them to a' transmission line. I

4. A system in accordance with claim 2 in which the cable is loaded withmagnetic material characterized by the provision of means such as afilter at the transmitting end for suppressing a band of at'leastseveral hundred cycles of the lower frequency waves of speech. 5. Asystem in accordance with claim 2, in which the cable is inductivelyloaded, characterized by the provision of means such as a filter at thetransmitting'end for suppressing a band of frequenciesof several hundredcycles of the higher frequency waves of speech. 0

6. A system in accordance with claim 2, in which the cablev isinductively loaded,

characterized by the provision of means such as a filter at thetransmitting end for suppressing a band of at least several hundredcycles of the lower andupperv frequencies waves of speech.

7. The method of transmitting speechfrequency currents over a submarinecable, which comprises selectively attenuating and amplifying thecurrents of speech frequency, before impressing them on the cable, insuch a manner that this frequency-amplitude characteristic afteramplification and attenuation is the converse of the cable transmissioncharacteristic modified in a manner converse to the amplitude-frequencydistribution of.

the energy of normal speech. q

8. A method in accordancewith the immediately foregoing claim, furthercharac} terized in this, that the frequency-amplitude characteristic ofthe speech transmitted to the cable is further modified proportionallyto the characteristic curve of the average vdis tribution of noiseenergy picked up along 9. Circuit arrangements for transmitting speechwave energy over a submarine cable having a certain characteristic ofaverage noise or interfering energy with frequency at the receiver,comprising a transmitter, a land line, means for suppressing severalhundred cycles of the lower frequencies of the speech spectrum, meansfor amplifying the upper portion of the residual frequency band at leastseveral times as much as the lower portion with an amplification whichincreases toward the upper end of the upper portion,

whereby the level of the received speech energy is substantiallyuniformly above the level of the received interfering energy.

10. A transoceanic cable telephone system comprisinga source of speechwaves, amplifying means and amplitude-frequency dis-' torting means atthe transmitting end, a cable, a receiver including amplifying means andamplitude-frequency distorting means, said system being characterized inthis, that the amplifying and amplitude-frequency distorting means atthe transmitting end impress upon the cable a maximum amplitude ofenergy in theifrequency range about 1800 to 2500 cycles, and theamplifying and ampli tilde-frequency distorting means at the receivingend impress upon the receiving device a maximum amplitude of energy inthe range around 1000 to 1200 cycles.

11. A speech transmission system comprising a line having a highlyvariable attenuationwith frequency, means at the transmitting endforproducing a wave derived from normal speech and of such nature thatafter transmission over said line the various re- 'ceived componentswithin a wide band have a frequency-amplitude relation much more nearlyuniform than would be the case for, transmission of normal speech, andmeans at the receiving end for converting the received wave into a wavemore nearly approximating the normal amplitude frequency relation ofspeech than that received.

In'witness whereof we hereunto subscribe our names this 9th day ofFebruary, 1929.

. OLIVER E. 'BUGKLEY.

RALPH V. L. HARTLEY.

the cable and received at the receiving end thereof. 7

